Usual interstitial pneumonia (UIP) is a common chronic interstitial pneumonitis. It can occur idiopathically (I-UIP) or in the setting of systemic connective tissue disease (CTD-UIP). Some studies suggest that CTD-UIP has a better prognosis than I-UIP. The histologic differences between CTD-UIP and I-UIP are not clearly defined.
The purpose of this study was to evaluate histologic criteria that may differentiate CTD-UIP from I-UIP, including fibroblastic foci (FFs), lymphoid aggregates (LAs), and the presence of nonspecific interstitial pneumonia pattern.
Thirty-five patients with histologic diagnoses of UIP were identified (27 biopsies [77%]; 8 explants [23%]). Biopsy slides were scanned and analyzed quantitatively for FF size, FF area, LA size, and LA area. Biopsy and explant slides were examined qualitatively for the presence of a nonspecific interstitial pneumonia pattern in areas away from UIP fibrosis. Results.-Of 27 biopsies, the number and size of FFs in CTD-UIP were smaller than they were in I-UIP. The number and size of LAs were larger in patients with rheumatoid arthritis than they were in patients with I-UIP. There was no interobserver variability among 3 pathologists using this quantitative system. Of 35 biopsies and explants, there was a higher prevalence of the nonspecific interstitial pneumonia pattern among patients with CTD-UIP than there was among patients with I-UIP (P = .005).
Patients with CTD-UIP had fewer, smaller FFs than did patients with I-UIP, and patients with rheumatoid arthritis-UIP had more, larger LAs than did patients with I-UIP. Of importance, the coexistence of UIP and the nonspecific interstitial pneumonia patterns was one of the most salient features in distinguishing CTD-UIP from I-UIP because CTD-UIP demonstrated an increased prevalence of multilobar, cellular, nonspecific interstitial pneumonia patterns in areas away from the UIP fibrosis.